Revisit and evaluate skiplists

We have skiplists in three locations:
* VPR: https://github.com/elastic/docs-tools/blob/master/versioned_plugins.rb [1]
* LSR: https://github.com/elastic/logstash/blob/master/rakelib/plugins-metadata.json [2]
* Another skiplist: https://github.com/elastic/docs-tools/blob/master/settings.yml [3]

Comments:
* [3] lives in docs-tools, and skips whole repos. (If we're skipping an entire repo, will adding repos here be all we need to skip a plugin?)
* [2] contains settings other than `"skip-list"`: `"default-plugins"` and sometimes ` "core-specs"`
* Do we have use cases where we want to skip a plugin for one but not both of [Logstash Reference](https://www.elastic.co/guide/en/logstash/current/index.html) and [Versioned Plugin Reference](https://www.elastic.co/guide/en/logstash-versioned-plugins/current/input-beats-index.html)?

ToDo:
- [ ] Figure out how these skiplists interrelate
- [ ] See if we can simplify
- [ ] Document best practices:
- [ ] https://github.com/elastic/ingest-dev/tree/master/logstash/logstash-docs-processes
- [ ] comments in code